A novel expperimental preparation will permit examination of reflex control of left ventricular contractility and related parameters by the aortic body chemoreceptors without direct effects of chemoreceptor stimuli on myocardial contractility or indirect effects via other reflex pathways. Aortic bodies of anesthetized, open chest dogs will be stimulated by reducing PO2 or raising CO2 in inspired gas, or by altering independently pH or adding nicotine to blood perfusing aortic arch. Myocardial contractility will be assessed from 1) left ventricular dP/dt max, 2) segment length changes measured with ultrasonic crystals, 3) ventricular function curves. Left coronary blood flow will be measured electromagnetically and its transmural distribution measured with radioactive microspheres. Myocardial O2 consumption will be measured by Fick principle. Independent variations in PO2, CO2, pH, and nicotine content of aortic blood will be made stepwise so that stimulus thresholds can be determined and stimulus-response relations defined. Levels of stimuli will also be varied simultaneously to assess combined effects on aortic body activity. Interaction between aortic body and other cardiac reflexes will be examined. The efferent limb of aortic body-cardiac reflex will be identified and studied with autonomic blocking drugs. This investigation will define fundamental properties of aortic body-cardiac reflex which may lead to a better understanding of its role during disturbances in arterial blood gases, such as occur clinically with pulmonary disease or cardiac failure.